In a fuel cell, a fuel gas containing hydrogen is supplied to an anode, and an oxidant gas containing oxygen is supplied to a cathode, so that, in the anode and the cathode, electrochemical reactions indicated by the formulae:H2→2H−+2e−  (1)(½)O2+2H−+2e−→H2O  (2)occur, and, in the whole of the cell, an electrochemical reaction indicated by the formula:H2+(½)O2→H2O  (3)proceeds. The chemical energy of the fuel is directly converted into an electrical energy, with the result that the cell can exert predetermined cell performance.
A fuel cell separator of the solid polymer electrolyte type or the phosphoric acid type in which such energy conversion is conducted is requested to be gas-impermeable, and also to be made of a material of high electrical conductivity in order to improve the energy conversion efficiency. Conventionally, it is known that, as a material meeting the requirements, an electrically conductive resin is used. An electrically conductive resin is a complex which is configured by bonding graphite (carbon) powder by means of a thermosetting resin such as phenol resin, or a so-called bondcarbon (resin-bonded carbon) compound. A technique is conventionally employed in which a fuel cell separator is produced by loading the bondcarbon compound into a mold, and resin-molding into a predetermined shape in which ribs for forming fuel gas passages, oxidant gas passages, or coolant water passages are formed integrally on at least one face of a separator molded member.
In such a fuel cell separator which is resin-molded into the predetermined shape by using a bondcarbon compound, when the thermosetting resin is softened by heating during the resin molding process, part of the thermosetting resin oozes to the surface layer to form a thin resin layer on the surface of the separator molded member. The thin resin layer is naturally formed also on the surfaces of the ribs for forming passages and functioning as a contact surface with an electrode in a product (separator).
The thin resin layer which is formed on the surface of the separator molded member in this way is an electrical insulating layer, and does not exhibit conductivity. As a whole of the separator, therefore, the conductivity is lowered, and the specific resistance is increased. Moreover, also the contact resistance with an electrode is increased by the presence of the thin resin layer which is formed on the surfaces of the ribs. The contact resistance with an electrode on the surface of each rib which is increased by the formation of the thin resin layer is larger by one digit than the specific resistance of the whole separator which is similarly increased by the formation of the thin resin layer. The increase of the contact resistance more strongly affects the internal resistance of the fuel cell which is the sum of the specific resistance and the contact resistance. In order to improve the performance and efficiency of the fuel cell, therefore, it is requested to reduce the contact resistance of the surfaces of the ribs with an electrode, as much as possible.
As means for satisfying such a request, conventionally, the following means have been proposed. For example, Japanese Patent Application Laying-Open No. 11-204120 discloses means for polishing away the surfaces of ribs to physically remove a thin resin layer, and Japanese Patent Application Laying-Open No. 11-297338 discloses means for immersing a separator in, for example, a strongly acidic solution into which one or two or more of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, and the like are mixed, to acid-treat the surface, whereby the surface roughness of the surfaces of the ribs is adjusted to Ra=0.1 μm to 10 μm so as to reduce the contact resistance.
In the case of the former one of the means which have been conventionally proposed, i.e., the resin layer physically removing means based on polish removal of the surfaces of the ribs, it is technically very difficult to remove only the thin resin layer, and hence also graphite particles which contribute to the conductivity are easily removed away together to reduce the amount of graphite particles in the surfaces of the ribs. As a result, the graphite density of the contact surface with an electrode is reduced, so that the contact resistance cannot be sufficiently lowered.
In the case of the latter means, i.e., the means for immersing a separator in an acidic solution to acid-treat the surface, if the surface roughness is not adjusted to the above-mentioned specific range, the contact resistance tends to be increased on the contrary. Therefore, the treatment itself requires a very sophisticated technique and much labor, and hence the means is not preferable from the viewpoints of the production efficiency of a separator, and the production cost. Furthermore, the means has problems in that, during the acid treatment, the acidic solution reacts with graphite particles to form graphite oxide and reduce free electrons, and, because of this and the like, the conductivity inherent in graphite particles is easily impaired, and that phenol resin itself is acid resistant and is not efficiently eroded, and hence the specific resistance of the whole separator is increased and the contact resistance with an electrode cannot be sufficiently lowered.